Marijuana Blog

Heavy marijuana use is associated with cognitive decline in about 5% of teens, according to a new study, which suggests that the heaviest users could lose 8 IQ points.

In the report, published in the Proceedings of the National Academy of Sciences, research conducted in New Zealand showed that teens who started smoking marijuana before age 18 and were diagnosed as being addicted to cannabis by age 38 experienced an IQ drop in early adulthood. But users who began smoking after age 18—even if they used heavily— did not show a significant decline.

“The effect of cannabis on IQ is really confined to adolescent users,” says lead author Madeline Meier, a postdoctoral researcher at Duke University, “Our hypothesis is that we see this IQ decline in adolescence because the adolescent brain is still developing and if you introduce cannabis, it might interrupt these critical developmental processes.”

The authors followed 1037 children born in the town of Dunedin, New Zealand in 1972 and 1973, virtually every child. They defined adolescent use as at least weekly use before turning 18. In looking at the relationship between marijuana use and IQ, they controlled for factors like years of education, schizophrenia and use of alcohol or other drugs that might also have an effect on IQ. While education weakened the relationship, it still did not eliminate it.

Researchers also had family members and friends of the participants confidentially rate them on attention and memory skills and those who had lost IQ points showed problems in these areas. Meier notes that an 8 point decline in IQ for someone with average intelligence (an IQ score of 100; the 50th percentile) would move that person down to the 29th percentile. “It’s fairly substantial but it does depend on where you start out,” she says.